As is well known, optical fibers are coated during drawing with a polymeric coating intended to protect them from the outside environment. Such a coating comprises, in general, two layers: an inner layer (primary coating), which is relatively soft, and an outer layer (secondary coating) which is more rigid. This structure allows protection of the fiber itself from chemical agents and mechanical actions which could alter its characteristics, e.g. induce attenuation due to microfractures or even cause the fiber to break. However, the coating itself has an influence on the overall behavior of the fiber from the optical and mechanical point of view. This influence depends not only on the type of coating material, but also on the coating application and polymerization processes. To fully characterize the fiber, it is therefore important to know the characteristics of the coating, and in particular the viscoelastic characteristics (viscous modulus, elastic modulus, glass transition temperature . . . ).
The techniques proposed until now to determine these characteristics are based on the analysis of the behavior of isolated, film-shaped polymer samples. The characterization of optical fiber coatings using these techniques is described, for instance, in the papers "Designing an optical fiber dual coating system for loose tube and ribbon cable long line and local loop applications", by R. J. Overton et al., Proceedings of 42nd International Wire and Cable Symposium, pages 701-707, and "Rheological Characterization of Coatings for Fabrics and Fibers", by C. L. Rohn, Clemson University Conference on Coated Fabrics, 2-3 May 1989, Clemson (USA), paper number 671.
In these methods, the sample undergoes a periodically variable deformation, applied by means of a suitable instrument (rheometer) and the resistant torque or stress is measured. From the measured quantity, the elastic modulus, the viscous modulus, the glass transition temperature, etc., of the sample are calculated; by extrapolating the data thus found, the behavior with time of the coated fiber is determined.
It is clear that, due to the different geometric characteristics, the response of a film to a mechanical deformation is very different from that of a cylinder, in particular because of the existence in the former case of boundary effects. Moreover, this type of measurement does not take into account the influence of the fiber on the coating due to adhesion forces. Simple extrapolation of the measurements on the isolated sample is not sufficient to provide reliable data on the behavior of a coating in operating conditions.